Pour the milk into your slow cooker insert. Cover and cook on low for 2 1/2 hours. Allow mixture to sit for another 3 hours with the lid on.
Add yogurt to the milk and gently stir through, replace the lid. Wrap a bath towel around slow cooker and the lid. (Blocking out any light and keeping the cooker warm.) Leave the yogurt mixture in the slow cooker for 8-12 hours.
After 12 hours you will have slightly runny yogurt. Leave for a few hours longer if you want a thicker yogurt. For Greek-style yogurt, line a strainer with cheesecloth or with muslin. Place the strainer over a bowl (to catch excess whey) pour mixture into strainer and strain for at least one hour. The longer you strain, the thicker the yogurt will be.
Store in the fridge once desired consistency is achieved.
Tip: freeze the excess whey in ice cube trays and use for your next batch of yogurt. Simply remove from the freezer and allow to return to room temperature before adding to yogurt. Alternatively, add the whey to smoothies for a nice dose of probiotics.
Serving suggestions: sprinkle your yogurt with granola, cinnamon, brown rice syrup, cacao nibs or coconut flakes.
This recipe is from the new I Quit Sugar Slow Cooker Cookbook.